In this paper, the temperature detection of the reed inside RF coaxial switch based on the principle of fiber grating is studied， and the polyimide coated fiber grating is used. After the temperature calibration, the linearity of the fitting straight line using the external temperatures and the center wavelengths of the fiber grating is 99.914%. The center wavelength drift is 10.39 pm for every 1 ℃ rise. The calibrated fiber grating is placed at the reed of the RF switch for the temperature measurement experiment. The experimental results show that when the internal temperature of the RF switch reaches 300 ℃, the theoretical center wavelength deviation error of fiber grating is 0.02 nm, and the corresponding repeated temperature measurement error is 2 ℃, which provides an effective method for detecting the reed temperature inside the RF coaxial switch.